Virgin LHR to Miami and Transit

Hi all
Thanks for the help with our Dec Antigua flights. Before then we are flying to Cancun in August - UC with VA to Miami and then Mexicana to Cancun and reverse inbound. Outbound we have 3 hours betweeen arrival and departure in Miami and inbound have 2hr 45mins. Have any of you transited in Miami before and if so, have we allowed enough time and also any tips to help us get through quickly in Miami as I understand we need to clear immigration and customs and check back in again rather than just transit through airside?
Thanks again
Thanks for the help with our Dec Antigua flights. Before then we are flying to Cancun in August - UC with VA to Miami and then Mexicana to Cancun and reverse inbound. Outbound we have 3 hours betweeen arrival and departure in Miami and inbound have 2hr 45mins. Have any of you transited in Miami before and if so, have we allowed enough time and also any tips to help us get through quickly in Miami as I understand we need to clear immigration and customs and check back in again rather than just transit through airside?
Thanks again